1. Field of the Invention
This disclosure is related to the field of oil recovery from mixtures including oil, water, and contaminants. Specifically to oil recovery from mixtures of water from oil refinery or other hydrocarbon handling operations.
2. Description of the Related Art
Oil refining in order to convert crude oil into products such as gasoline, diesel, and the petroleum-based materials common in modern society is a complicated and generally messy process. Crude oil is not a simple material but a complex mixture including many different hydrocarbons and other materials whose exact composition can vary widely from oil field to oil field. The refinery process, therefore, is often focused on removing contaminants from the basic crude oil to allow the hydrocarbon compounds to be refined into more useful compounds. These contaminants are often water, and solids which may be suspended in the water or oil. Specifically crude oil having less than 1% Basic Sediment and Water (BS&W) is generally considered ready for processing regardless of its particular hydrocarbon composition.
There are a number of steps which occur in the refining including, but not limited to, desalting and dewatering which produce a water-based effluent. This effluent is generally stored in tanks, ponds, or other storage facilities until it can be clarified and cleaned and the water present therein can be returned to the environment. Traditionally, the principle concern with this effluent mix was the safe disposal of contaminants (including remaining hydrocarbons or waste oil) and clarification of water for return or reuse.
Refinery waste streams, and, in fact, many types of wastewater from other industries which routinely handle hydrocarbon operations, are laden with both hydrocarbons such as, but not limited to crude oil, as well as a number of solid contaminants. This wastewater cannot be discharged to the environment as it can be generally toxic to water ecosystems, and most standard water treatment plants are unable to handle the specific materials produced in such hydrocarbon operations. Specifically, they often cannot remove the waste oil that is present.
In addition to wastewater streams from refineries including waste oil, waste oil is also generated in other processes. For example operations of automotive or other engines generally utilize oil (usually called engine oil, motor oil, or crankcase oil) in order to lubricate parts. This oil needs to be periodically replaced as the oil will, over time, become contaminated with particulates (ash) which can interfere with engine operation. Specifically, the engine oil is designed to capture and hold these particulates in suspension to prevent them from being contacted by moving parts of the engine and potentially causing damage. However, as more of them become captured, the oil will thicken and its ability to lubricate, and retain additional particles, is decreased.
Traditionally, used engine oil was simply disposed of. However, the oil is not fundamentally damaged by the lubrication process (in most cases) and can be reused if the oil can be cleaned to remove the captured particulates, additives which may be added to improve its function, and water which generally also contaminates used engine oil through exposure over time. The oil can then be re-refined into useful products.
Traditional systems for separating oil from water and solids utilize well known properties of the materials. Namely, hydrocarbons are generally insoluble in water and will float on top of water. Thus, traditional separation of water from hydrocarbons in all sorts of tanks relies on agitation, followed by simple gravity separation. This type of wastewater treatment effectively places the wastewater in a vessel, agitates it, and then allows it to phase so solids settle to the bottom of the tank, water separates to the middle, and hydrocarbons go to the top. The water and hydrocarbons can then be separated by pumping from differential levels. Alternatively, wastewater can be centrifuged, cycloned, or otherwise handled in a manner which utilizes the same properties in a slightly different manner.
While these processes are generally sufficiently good at segregating oil, water, and solids to allow for the separated hydrocarbons (oil) to be processed, the water to be cleaned using known processes, and solids to be collected, it is not a process which is particularly effective at capturing the oil present in the wastewater. Specifically, while the existing basic agitation processes can separate easily separable components, there is generally a fourth layer present (often called the “rag” layer) which includes, for the most part, the hydrocarbons, solids, and other materials (with certain limited exceptions) mixed together. This mixture was simply considered unusable waste or was utilized in alternative processes not designed to capture the hydrocarbons for processing. For example, the wastes could be burned to produce electricity or could be used as part of a raw crude feed. Further, oil is also commonly trapped in the solids layer resulting in these including a large amount of oil and potentially needing to be treated as hazardous.
The reason that the rag and solid layers were uninteresting is that processes for wastewater treatment were generally focused on cleaning the water in the system and water reclamation. While capture of easily segregatable oil was seen as beneficial since it could be used as refinery feedstock, the primary purpose of removing it was to assist in cleaning the water, not in capturing the oil. This lack of focus on oil reclamation and therefore a relatively large percent of valuable hydrocarbons (often greater than 75% of crude oil originally in the wastewater stream) was simply disposed, generally by the rag and solid mix being used as an inefficient combustion source.
When crude oil prices are relatively low, this is not a particularly big issue as lost hydrocarbon material is simply a small cost of doing business in the hydrocarbon area. However, as oil prices go up, and as there becomes increasing public and environmental concern for disposal of potentially hazardous (and potentially useable) materials in landfills and other waste disposal sites, interest in obtaining additional amounts of “waste oil” from within waste streams and allowing that oil to be processed and used is increasing. Further, combustion of oil, water, and contaminant mixtures can result in the release of undesirable materials into the air (such as sulfurs and metals), which presents a further concern.
Further, most wastewater (and oil) storage vessels also require periodic cleaning. Crude oil is generally sticky and will adhere to most surfaces. Therefore, water is often used to clean tanks, pipes, and other vessels where oil has been present and this water also requires clarification in order to remove the oil. Wastewater can also be generated from oil cleanup activities such as tanker spills and from oil drilling operations. Thus, there can be significant amounts of wastewater present in the handling of hydrocarbons which are not only generated as part of operations, but are created through the actions of cleaning or accidents. These wastewaters, again, have generally been treated for clarification of water and the loss of valuable oil was simply seen as a cost of doing business.